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The effect of D-region absorption on the forward-scatter radio meteor amplitude distribution index
Journal of Atmospheric and Terreetrial Physics, 1973, Vol. 35, pp. 2277-2282. Pergamon Press. Printed in Northern Ireland
SHORT PAPER The effect of D-region absorption on the forward-scatter radio meteor amplitude distribution index J. JONES and J. G. COLLIXS apartment
of Physics and Centre for Radio Science, The University of Western Ontario, London, Ontario, Cans& (Received 5 March 1973; in rev&d
form12 April 1973)
Ab&r&-The ~vesti~tion of sporadic ~o-rne~~ using a 49-m.& fo~~d-~t~r circuit with 8 baseline of approximately 1600 km during mid-July 1971, indicated 8 strong diurnal v&ation in the amplitude distribution index 9’. We have attributed this variation to absorption in the D-region and a method is illustrated by which the 8bsorption may be determined. The results obt&ned by this method agree very well with riometer determinations for mid-July.
INTRODUCTION THE STUDY of radio meteors using forward-scatter systems has been carried out for many years (see, for example, VOGAN and CAMPBELL,1957). While many aspects
of the data collected with this method are consistent with those obtained using other techniques, some difficulties have arisen in connection with the mass distribution of sporadic meteors deduced from fo~ard-scatter data (JONES and COLLINS, 1970). In this article we show that the forward-scatter radio meteor data can be strongly affected by D-region absorption. One of the parameters that characterixes a meteor population is the mass distribution index s which is defined by the equation
tw =
GTWa
dnz
(1)
where diV is the number of meteoroids in the mass range m to m + dm, and c is a constant. The mass of a meteoroid is difficult to measure directly. It is usually inferred from the luminosity or degree of ionization of the meteor train which subject to certain restrictions, is proportional to the initial meteoroid mass. In this paper we shall restrict our discussion to radio techniques for the de~rmination of the mass distribution index, which rely on the reffection of radio waves from the ionized column which is formed when a meteoroid ablates in the Earth’s atmosphere. The theory of the reflection of radio waves by meteor trains has been examined extensively by several authors, notably by KAISER and CLOSS(1952) who showed that provided the electron line density is sufficiently low, the electrons scatter substantially dependently of each other. In this approximation the amplitude A of an echo from a meteor train is proportional to the electron line density a, and hence the initial meteoroid mass. According to Kaiser and Closs, the electrons in trains having electron line densities in excess of 2.4 x 1Ol2 cm-l no longer scatter independently and in this regime the echo amplitude is proportional to ~$4. 12
2277
2278
J. JONES and J. G. COLLINS
The echo amplitude distribution index s’ is defined by dN = const A-“’ dA where dN is the rate of echoes in the amplitude range A to A + dA. to 8 by the equations
(2) 8’ is related
cc< 2.4 x lOI cm-l
8'= 8 8'= 48 -3
a > 2.4 x lOI cm-l
and thus s’ = f(A), wheref(A) varies most rapidly with A for electron line densities close to u = 2.4 x 1012 cm-l. In back-scatter systems of high sensitivity, slight changes in the sensitivity due to absorption in the D-region have little effect. This is due to the fact that f(A) is substantially constant when cc g 2.4 x lOI cm-l and also because the high elevation of the main beam of most back-scatter antenna systems results in only a very small change in the sensitivity due to absorption. In spite of this BALDWIN and KAISER (1962) have been able to estimate absorption coeflicients from the variation in the rates of back-scatter radio meteor echoes. For a low sensitivity (a e 1012 cm-l) forward-scatter system with a long base line, the changes in sensitivity are enhanced due to the low elevation of the antenna beam. CHUet al. (1966) have measured absorption coefficients from the ratio of meteor echo amplitudes observed at two frequencies on the same forward-scatter path. In the present work the changes in the sensitivity of the system due to D-region absorption are deduced from the changes in the echo amplitude distribution index. EXPERIMENTAL PROCEDURE The forward-scatter system used for this experiment consisted of a 49*95-mHz transmitter located in Winnipeg (Manitoba) with the receiver in London (Ontario). The transmitting and receiving antennae were oriented to illuminate the meteor region midway between London and Winnipeg. The observations were made 24 hr a day on 14-15, 19-20 and 27-28 July 1971, when meteor shower activity was expected to be low. All meteor echoes which did not show a sharp leading edge were rejected from the analysis as these were most probably the result of distortion of overdense trains by wind shears. The records were analyzed utilising a Clevite Brush Mark 260 pen recorder which had a frequency response up to 60 Hz so that no bias in favour of long duration echoes was expected due to this cause. RESULTSAND ANALYSIS Figure 1 shows the diurnal variation of the mean hourly rate of meteor echoes. The data shows a maximum activity close to sun-rise which is due to the wellknown Apex Effect (MCKINLEY, 1961). In Fig. 2 we have plotted on a log-log graph the rate N(A) of echoes with amplitudes in excess of A. Integration of equation (2) yields N(A)
cc A1-s’.
Ifs’ were constant, a log-log plot of N(A) against A should be a straight line. Since this is not so for our data, we have calculated s’ from adjacent points on Fig. 2
D-region absorption
on the forward-scatter
II I8
distribution
I
I
I
I
I
1
I
I
I
I
I
20
22
24
02
04
06
08
IO
12
14
16
Local
Fig.
radio meteor amplitude
time
ot
mid-point
of
scatter
geometry
1. Mean hourly rate of meteor echoes with amplitudes
greater than A.
I
‘+ \ I
\
+ \
+
a
H
I(
\
log (A), Fig.
0.4
orbitrory
0.60.8
I.0
units
2. The meteor echo rate ea a function
index
of echo amplitude.
2279
J. JONES and J. G. COLLINS
2280
in order to get 5’ as a function of A, which is shown in Fig. 3. A priori there is no reason to suppose that there is a linear relation between s’ and A but Fig. 3 indicates that the relation is fairly linear over an appreciable amplitude range, although we cannot extrapolate with any confidence to the value of s’ for zero amplitude. Figure 3 enables us to relate the changes in the amplitude distribution index to changes in the sensitivity of the forward-scatter circuit and hence absorption along the ray path. With this in mind we have plotted Fig. 4 which shows the diurnal variation of s’ calculated from the two most sensitive levels. To convert changes in sensitivity to absorption coefficients we must consider the geometry of the system, which is shown in Fig. 5. It is easily shown that the path length Df in the absorbing region is given by Df N D,L/(h + h2/8R) where D, is the vertical extent of the absorbing region, L is the distance between the transmitter and receiver and R is the radius of the Earth. With .L = 1.6 x lo3 km,
R = 6.3 x lo3 km
and
h = 100 km,
we obtain D, = 10.6 D,. We can now express our results in terms of the absorption over a one-way vertical path. On the assumption that the minimum absorption is zero we have found the absorption coefficient from the measured value of s’ and hence we have obtained the decibel scale in Fig. 4. DISCUSSION AND CONCLUSIONS The diurnal variation of the absorption coefficient for the D-region deduced from these results compares well with that found by LTJSICNAN(1960), who, using a 27.5-MHz riometer, found the D-region absorption for the summer months of 1958 at moderate latitudes to be about 1 dB in the daytime. Probably our values are, if anything, a little high, but the uncertainty is due mainly to ignorance of the proper 403.8 3.6 3432S’
30282624-
222.0 I IO Meon
Fig.
3. The variation
I 2 .o amplitude
of echo amplitude
I 3.0
,
I 40 arbitrary
distribution
I 50 units
index with echo amplitude.
D-region absorption on the forward-scatter radio meteor amplitude distribution index
2281
I,0
I 0.8 06
dB
0.4
S’
0.2 0
XII I8 20 22
24 02 04 06 08
Load time at mid-point
Fig.
4.
i0 12 14 16 of scatter geometry
The diurnal variation of the amplitude distribution index s’.
Fig. 5. The forward-scatter geometry.
functional relation between 8’ and A. This can be improved by a detailed calculation of the scattering of radio waves from meteor trains and also by the analysis of more data. As a method for the deter~nation of the D-region absorption coe~~ient the technique described here is very straight forward and lends itself to automatic operation since the s’ values can be deduced by comparing the rates of echoes observed at two pro-determined amplitude levels. Its only drawback might be difficulty of interpretation in periods of high meteor shower activity.
2282
J. JONESand J. G. COGS
The present method is simpler than that used by CHU et al. (1966) in that only
instead of two. It is of interest to note that since the method relies on the change in the reflection process from underdense to overdense echoes, increasing the sensitivity of the equipment to increase the echo rate actually makes the method less sensitive to any changes in absorption since 8’ varies most rapidly with echo amplitude in the reflection transition region. Finally the results obtained here draw attention to the fact that for forwardscatter systems the changing reflection mechanism must be taken into account when trying to deduce mass distribution indices from echo amplitude distribution indices. We feel that in the case of forward-scatter systems the sensitivity is likely to be considerably less than the optimum calculated on the assumption of a perfect ground plane, since at low angles of elevation the effect of terrain irregularities becomes very pronounced. Thus although the system used in this experiment had a limiting sensitivity of about a = 2 x lOI cm-l under ideal conditions, we are of the opinion that since the antenna installation was located on the slope of a valley, the degrading effect of the terrain irregularities was to reduce the sensitivity by a large factor. We feel that this reduced sensitivity of forward scatter systems is the main cause of the high values of the mass exponent reported by some workers (JONES and COLLINS, 1970; TRISEOVA and SAMARDZIYEV, 1967; ESHLEMBN and one frequency is used
MANNINQ, 1954). A&nowledgement--The program of which this work forma a part was supported by a grant from the National Research Council of Canada. REFERENCES BALDWIN J. P. and KAISER T. R. CHU Y., VOQAN E. L. md FORSYTH P. A. ESHLEMAN V. R. and MANNING L. A. JONES J. and COLLINS J. G. KAISER T. R. and CLOSS R. L. LUSIGNAN B . MCKINLEY D. W. R.
1964 1966
TRISKOVA L. and SA~ZTYEV D. T. VOQANE. L. and CAbfPBELL L. L.
1967 1957
1954
1970 1952 1960 1961
Mon. Not. R. a&on. Sot. 129, 86. Can. J. Phye. d&2173.
Pmt. IRE 42,630. Can. J. Phya. 48,2585. Phil. Mazg. 43, 1. J. geophys. Rea. 65,3896. Meteor Science ad Engineering. Hill, New York. &omgn. Am. 7.2, 237. Can. J. Phya. 35, 1176.
McGraw-
SHORT PAPER The effect of D-region absorption on the forward-scatter radio meteor amplitude distribution index J. JONES and J. G. COLLIXS apartment
of Physics and Centre for Radio Science, The University of Western Ontario, London, Ontario, Cans& (Received 5 March 1973; in rev&d
form12 April 1973)
Ab&r&-The ~vesti~tion of sporadic ~o-rne~~ using a 49-m.& fo~~d-~t~r circuit with 8 baseline of approximately 1600 km during mid-July 1971, indicated 8 strong diurnal v&ation in the amplitude distribution index 9’. We have attributed this variation to absorption in the D-region and a method is illustrated by which the 8bsorption may be determined. The results obt&ned by this method agree very well with riometer determinations for mid-July.
INTRODUCTION THE STUDY of radio meteors using forward-scatter systems has been carried out for many years (see, for example, VOGAN and CAMPBELL,1957). While many aspects
of the data collected with this method are consistent with those obtained using other techniques, some difficulties have arisen in connection with the mass distribution of sporadic meteors deduced from fo~ard-scatter data (JONES and COLLINS, 1970). In this article we show that the forward-scatter radio meteor data can be strongly affected by D-region absorption. One of the parameters that characterixes a meteor population is the mass distribution index s which is defined by the equation
tw =
GTWa
dnz
(1)
where diV is the number of meteoroids in the mass range m to m + dm, and c is a constant. The mass of a meteoroid is difficult to measure directly. It is usually inferred from the luminosity or degree of ionization of the meteor train which subject to certain restrictions, is proportional to the initial meteoroid mass. In this paper we shall restrict our discussion to radio techniques for the de~rmination of the mass distribution index, which rely on the reffection of radio waves from the ionized column which is formed when a meteoroid ablates in the Earth’s atmosphere. The theory of the reflection of radio waves by meteor trains has been examined extensively by several authors, notably by KAISER and CLOSS(1952) who showed that provided the electron line density is sufficiently low, the electrons scatter substantially dependently of each other. In this approximation the amplitude A of an echo from a meteor train is proportional to the electron line density a, and hence the initial meteoroid mass. According to Kaiser and Closs, the electrons in trains having electron line densities in excess of 2.4 x 1Ol2 cm-l no longer scatter independently and in this regime the echo amplitude is proportional to ~$4. 12
2277
2278
J. JONES and J. G. COLLINS
The echo amplitude distribution index s’ is defined by dN = const A-“’ dA where dN is the rate of echoes in the amplitude range A to A + dA. to 8 by the equations
(2) 8’ is related
cc< 2.4 x lOI cm-l
8'= 8 8'= 48 -3
a > 2.4 x lOI cm-l
and thus s’ = f(A), wheref(A) varies most rapidly with A for electron line densities close to u = 2.4 x 1012 cm-l. In back-scatter systems of high sensitivity, slight changes in the sensitivity due to absorption in the D-region have little effect. This is due to the fact that f(A) is substantially constant when cc g 2.4 x lOI cm-l and also because the high elevation of the main beam of most back-scatter antenna systems results in only a very small change in the sensitivity due to absorption. In spite of this BALDWIN and KAISER (1962) have been able to estimate absorption coeflicients from the variation in the rates of back-scatter radio meteor echoes. For a low sensitivity (a e 1012 cm-l) forward-scatter system with a long base line, the changes in sensitivity are enhanced due to the low elevation of the antenna beam. CHUet al. (1966) have measured absorption coefficients from the ratio of meteor echo amplitudes observed at two frequencies on the same forward-scatter path. In the present work the changes in the sensitivity of the system due to D-region absorption are deduced from the changes in the echo amplitude distribution index. EXPERIMENTAL PROCEDURE The forward-scatter system used for this experiment consisted of a 49*95-mHz transmitter located in Winnipeg (Manitoba) with the receiver in London (Ontario). The transmitting and receiving antennae were oriented to illuminate the meteor region midway between London and Winnipeg. The observations were made 24 hr a day on 14-15, 19-20 and 27-28 July 1971, when meteor shower activity was expected to be low. All meteor echoes which did not show a sharp leading edge were rejected from the analysis as these were most probably the result of distortion of overdense trains by wind shears. The records were analyzed utilising a Clevite Brush Mark 260 pen recorder which had a frequency response up to 60 Hz so that no bias in favour of long duration echoes was expected due to this cause. RESULTSAND ANALYSIS Figure 1 shows the diurnal variation of the mean hourly rate of meteor echoes. The data shows a maximum activity close to sun-rise which is due to the wellknown Apex Effect (MCKINLEY, 1961). In Fig. 2 we have plotted on a log-log graph the rate N(A) of echoes with amplitudes in excess of A. Integration of equation (2) yields N(A)
cc A1-s’.
Ifs’ were constant, a log-log plot of N(A) against A should be a straight line. Since this is not so for our data, we have calculated s’ from adjacent points on Fig. 2
D-region absorption
on the forward-scatter
II I8
distribution
I
I
I
I
I
1
I
I
I
I
I
20
22
24
02
04
06
08
IO
12
14
16
Local
Fig.
radio meteor amplitude
time
ot
mid-point
of
scatter
geometry
1. Mean hourly rate of meteor echoes with amplitudes
greater than A.
I
‘+ \ I
\
+ \
+
a
H
I(
\
log (A), Fig.
0.4
orbitrory
0.60.8
I.0
units
2. The meteor echo rate ea a function
index
of echo amplitude.
2279
J. JONES and J. G. COLLINS
2280
in order to get 5’ as a function of A, which is shown in Fig. 3. A priori there is no reason to suppose that there is a linear relation between s’ and A but Fig. 3 indicates that the relation is fairly linear over an appreciable amplitude range, although we cannot extrapolate with any confidence to the value of s’ for zero amplitude. Figure 3 enables us to relate the changes in the amplitude distribution index to changes in the sensitivity of the forward-scatter circuit and hence absorption along the ray path. With this in mind we have plotted Fig. 4 which shows the diurnal variation of s’ calculated from the two most sensitive levels. To convert changes in sensitivity to absorption coefficients we must consider the geometry of the system, which is shown in Fig. 5. It is easily shown that the path length Df in the absorbing region is given by Df N D,L/(h + h2/8R) where D, is the vertical extent of the absorbing region, L is the distance between the transmitter and receiver and R is the radius of the Earth. With .L = 1.6 x lo3 km,
R = 6.3 x lo3 km
and
h = 100 km,
we obtain D, = 10.6 D,. We can now express our results in terms of the absorption over a one-way vertical path. On the assumption that the minimum absorption is zero we have found the absorption coefficient from the measured value of s’ and hence we have obtained the decibel scale in Fig. 4. DISCUSSION AND CONCLUSIONS The diurnal variation of the absorption coefficient for the D-region deduced from these results compares well with that found by LTJSICNAN(1960), who, using a 27.5-MHz riometer, found the D-region absorption for the summer months of 1958 at moderate latitudes to be about 1 dB in the daytime. Probably our values are, if anything, a little high, but the uncertainty is due mainly to ignorance of the proper 403.8 3.6 3432S’
30282624-
222.0 I IO Meon
Fig.
3. The variation
I 2 .o amplitude
of echo amplitude
I 3.0
,
I 40 arbitrary
distribution
I 50 units
index with echo amplitude.
D-region absorption on the forward-scatter radio meteor amplitude distribution index
2281
I,0
I 0.8 06
dB
0.4
S’
0.2 0
XII I8 20 22
24 02 04 06 08
Load time at mid-point
Fig.
4.
i0 12 14 16 of scatter geometry
The diurnal variation of the amplitude distribution index s’.
Fig. 5. The forward-scatter geometry.
functional relation between 8’ and A. This can be improved by a detailed calculation of the scattering of radio waves from meteor trains and also by the analysis of more data. As a method for the deter~nation of the D-region absorption coe~~ient the technique described here is very straight forward and lends itself to automatic operation since the s’ values can be deduced by comparing the rates of echoes observed at two pro-determined amplitude levels. Its only drawback might be difficulty of interpretation in periods of high meteor shower activity.
2282
J. JONESand J. G. COGS
The present method is simpler than that used by CHU et al. (1966) in that only
instead of two. It is of interest to note that since the method relies on the change in the reflection process from underdense to overdense echoes, increasing the sensitivity of the equipment to increase the echo rate actually makes the method less sensitive to any changes in absorption since 8’ varies most rapidly with echo amplitude in the reflection transition region. Finally the results obtained here draw attention to the fact that for forwardscatter systems the changing reflection mechanism must be taken into account when trying to deduce mass distribution indices from echo amplitude distribution indices. We feel that in the case of forward-scatter systems the sensitivity is likely to be considerably less than the optimum calculated on the assumption of a perfect ground plane, since at low angles of elevation the effect of terrain irregularities becomes very pronounced. Thus although the system used in this experiment had a limiting sensitivity of about a = 2 x lOI cm-l under ideal conditions, we are of the opinion that since the antenna installation was located on the slope of a valley, the degrading effect of the terrain irregularities was to reduce the sensitivity by a large factor. We feel that this reduced sensitivity of forward scatter systems is the main cause of the high values of the mass exponent reported by some workers (JONES and COLLINS, 1970; TRISEOVA and SAMARDZIYEV, 1967; ESHLEMBN and one frequency is used
MANNINQ, 1954). A&nowledgement--The program of which this work forma a part was supported by a grant from the National Research Council of Canada. REFERENCES BALDWIN J. P. and KAISER T. R. CHU Y., VOQAN E. L. md FORSYTH P. A. ESHLEMAN V. R. and MANNING L. A. JONES J. and COLLINS J. G. KAISER T. R. and CLOSS R. L. LUSIGNAN B . MCKINLEY D. W. R.
1964 1966
TRISKOVA L. and SA~ZTYEV D. T. VOQANE. L. and CAbfPBELL L. L.
1967 1957
1954
1970 1952 1960 1961
Mon. Not. R. a&on. Sot. 129, 86. Can. J. Phye. d&2173.
Pmt. IRE 42,630. Can. J. Phya. 48,2585. Phil. Mazg. 43, 1. J. geophys. Rea. 65,3896. Meteor Science ad Engineering. Hill, New York. &omgn. Am. 7.2, 237. Can. J. Phya. 35, 1176.
McGraw-